1. Field of the Invention
The present invention relates to a power supply circuit for driving a liquid crystal display (LCD). More particularly, the present invention relates to a power supply circuit used to generate different levels of voltage required to drive an LCD.
2. Description of the Prior Art
Portable electronic devices including cellular phones have become ubiquitous in recent years. Such portable electronic devices comprise an LCD panel as their display screen. The LCD panel is driven by a certain kind of power supply circuit which is well known in the art. More specifically, a multi-level power supply circuit is necessary to drive LCD panels which generates different levels of voltage The power supply circuit used for this purpose is hereinafter referred to as an “LCD drive power supply circuit”. It is noted that the term “LCD panel” as used herein is not limited to a particular LCD panel. Instead, it includes any kinds of similar LCDs which are used in portable electronic devices.
The multi-level LCD drive power supply circuit is incorporated within LCD drivers or LCD controller/driver ICs to generate necessary levels of voltage. Some conventional multi-level LCD drive power supply circuits use a separate power supply IC and resistors. However, the demand for lower power consumption and smaller driving circuit has been increasing as the LCD panels have found more and more applications in the portable electronic devices. A solution to meet such demand is to use a single chip LCD controller/driver on which an LCD drive power supply circuit is also incorporated.
A conventional single-chip LCD controller/driver with an on-chip LCD drive power supply circuit comprises a resistive voltage divider. The resistive voltage divider provides scaling of the peak voltage for LCD driving on desired voltage levels. However, charging and discharging the capacitive load of the panel would result in rounding of the waveform even when the voltage levels generated by the resistance division are used directly. Thus, the outputs of the resistive voltage divider are supplied to amplifiers where they are converted to have a low impedance. The low impedance waveforms are then supplied to multiplexers (drivers) where a certain level is selected in accordance with frame and display signals. The outputs of the multiplexer are used to drive segment electrodes and common electrodes configuring the panel.
In practice, the panel incorporates a plurality of electrodes and a plurality of corresponding outputs. For example, the panel may be configured of n number of segment electrodes and m number of common electrodes, which provides a display panel of n by m pixels. As is well known in the art, the common electrode is also referred to as a scanning electrode. Only one common electrode generates an output at the selected voltage level (selective output). The outputs from the remaining common electrodes are at the non-selected voltage levels. The segment electrodes generate outputs at the selected and unselected levels in synchronism with the selective output supplied from the common electrode. The outputs from the segment electrodes control the ON/OFF of the corresponding pixel because each cross point of the segment and common electrodes is a display pixel. It is noted that the voltage applied to the LCD is similar to the alternating current. Accordingly, the level selective/unselective modes for LCD driving fluctuate periodically according to a time period called a “frame”.
The voltage levels V1 to V5 for LCD driving are typically connected to capacitors C0 to C4 in order to stabilize the levels. The amplifiers A1 to A4 used to provide the levels for LCD driving are designed to reduce idling current and prevent shoot-through current as much as possible due to the capacitive load of the panel. Instantaneous switching of the load may result in fluctuation of the levels for a time period determined by the through rate of the amplifiers. This may adversely affect the display itself. With respect to the above, an external capacitor (bypass capacitor) may be added to each amplifier in order to eliminate any fluctuation of the levels if the through rate of the amplifier is not enough.
On the other hand, there have been increasing demands for lower power consumption and size reduction in the portable electronics field. In particular, it is required to eliminate any external capacitor and reduce the size of a chip as much as possible, in addition to reducing current consumption of a power supply circuit.
Japanese Patent Laid-Open No. 10-31200 discloses an LCD drive power supply circuit that meets the above-mentioned demands for lower power consumption, in which intermediate levels are used as the power supplies for level amplifiers with potential levels V3 and V4. Alternatively, Japanese Patent No. 2695981 (corresponding to EP0 479 304 B1) discloses an LCD drive power supply circuit having a configuration where the bias on an amplifier is turned OFF temporarily.
The LCD drive power supply circuits disclosed in the above specifications require lowerpower consumption. However, the chip size is increased due to the additional circuits. In addition, none of the above LCD drive power supply circuits are directed to the reduction of the circuit scale reduction.
Therefore, an object of the present invention is to provide an LCD drive power supply circuit with which the scale of the circuit can be reduced with a smaller number of components and in which switches and control signals are used to achieve a lower power consumption.